1. Field of the Invention
This invention relates to an electronic flash unit and more particularly to an improved trigger circuit for use with a flash-firing mechanism having a piezoelectric generator.
2. Description of the Prior Art
Flash-firing mechanisms which include a piezoelectric generator for triggering an electronic flash unit are known in the art. For example, U.S. Pat. No. 2,972,937 issued to C. G. Suits on Feb. 28, 1961, entitled FLASH APPARATUS discloses a piezoelectric crystal arranged to produce a high voltage impulse to fire a photoflash lamp in timed relation to light-unblocking movement of a camera shutter.
U.S. Pat. No. 3,782,258 issued to Boekkooi et al on Jan. 1, 1974, entitled IGNITION ARRANGEMENT FOR A DISCHARGE TUBE discloses a piezoelectric generator coupled to an electronic flash unit to initiate conduction of the xenon in the flashtube. In one embodiment disclosed therein, the piezo crystal output pulse is applied directly to the trigger electrode of the flashtube. In an alternative embodiment, the crystal pulse is applied to the gate electrode of an SCR which is turned ON to complete a conventional trigger circuit to energize the flashtube.
It has been found that the application of a piezo impulse directly to a flashtube trigger electrode as is done in the first embodiment disclosed in the Boekkooi et al patent does not always operate to fire the flashtube. This is despite the relatively high amplitude of the piezo pulse which may be several thousand volts. It has been theorized that due to the relatively short duration of this pulse, which may be on the order of approximately five (5) microseconds or less, the flashtube interelectrode resistance is not sufficiently reduced. When this results, the flash-firing capacitor fails to discharge through the flashtube and a flash does not occur.
In the other embodiment disclosed in the Boekkooi et al Patent, the conventional flash-trigger circuit comprises a trigger capacitor, which may be charged to several hundred volts, in parallel with a trigger electrode transformer, the primary winding of which is in series with the aforementioned SCR. This circuit avoids the problem referred to hereinbefore associated with a direct application of the piezo pulse to the trigger electrode since such a pulse is used merely to switch the SCR ON to render the trigger circuit conductive, not to trigger the flashtube. When the SCR conducts, the trigger capacitor discharges through the primary winding which under the influence of the transformer induces a voltage on the trigger electrode. This voltage, although possibly not having the peak amplitude of a piezo pulse, has a longer duration related to the oscillations produced by the circuit capacitance and the secondary winding of the trigger electrode transformer and has been found to fire the flashtube in a highly reliable manner not possible when the crystal signal is applied directly to the trigger electrode. However, the latter arrangement suffers from the disadvantage that it is complex and expensive owing to the fact that a piezoelectric generator is used in combination with a conventional trigger circuit. One of the advantages of a piezo generator is that it replaces a large number of electrical components which comprise a conventional electronic trigger circuit and this advantage is lost with the aforementioned combination